aged 50-90 years old. no musculoskeletal or peripheral or central nervous system disorders (other than ideopathic parkinson disease (ipd) or parkinsonism) that could significantly affect balance or gait .

capable of following directions. ipd subjects: uk brain bank criteria, i.e., bradykinesia and at least one of the following: rest tremor, muscular rigidity, and postural instability not cause by visual, vestibular, cerebellar or proprioceptive dysfunction. unilateral onset, response to levodopa.

parkinsonism subjects: gait charactized by slow short steps, shuffling gait and may be wide-based, with fog, postural instability.

inability to stand or walk for 2 min without an assistive device recent changes in medication excessive use of alcohol or recreational drugs, contraindications to mri scans (eg, claustrophobia, metal in body) intervention subjects will be excluded if: 1) participating in a vigorous exercise program more than 2 x/week, 2) a medical condition that contraindicates exercise participation.

parkinsonism subjects: ipd and parkinson plus syndromes such as progressive supranuclear palsy, multiple system atrophy, corticobasal syndrome, or cerebellar ataxia.

idiopathic pd subjects: same as above and deep brain stimulation electrodes. significant tremor that would interfere withmri scan.

control subjects: will be matched for age and gender to ipd and parkinsonism groups.